Much attention has recently been directed to materials having nonlinear optical effects, i.e., nonlinear properties between polarization and an electric field which is presented on reception of a strong photoelectric field such as a laser beam. Such materials are generally known as nonlinear optical materials and are described in detail, e.g., in David J. Williams (ed.), Nonlinear Optical Properties of Organic and Polymeric Materials, ACS Symposium Series 233, American Chemical Society (1983), and M. Kato & H. Nakanishi (eds.), Organic Nonlinear Optical Materials, CMC Co. (1985).
One of the applications of nonlinear optical materials is a wavelength modulation device utilizing second harmonic generation (SHG) ascribed to the secondary nonlinear effect and sum frequency wave and difference frequency wave. Nonlinear optical materials which have been used practically in this application include inorganic perovskite structures typically exemplified by lithium niobate. In recent years, .pi.-electron conjugated organic compounds having an electron donative group and an electron attractive group have been proved to exhibit greatly improved performance properties as nonlinear optical materials over the aforesaid inorganic materials. However, in the field of organic chemistry, difficulty arises in obtaining large single crystals with satisfactory quality. Moreover, achievement of nonlinear optical effects of the even number order (for example, SHG and secondary nonlinear optical effects represented by primary electro-optic effects) requires that the alignment of crystal molecules involves no inversion symmetry. In an attempt to avoid involvement of inversion symmetry in the molecular alignment, it has been proposed and studies have been conducted to introduce a hydrogen-bonding group or a chiral center, but fully satisfactory results have not necessarily been attained.
Hence, there has been proposed a method of blending a compound exhibiting a nonlinear optical response with a polymer to form a solid solution and, in order to agree with manifestation of the nonlinear optical effects of the even number order, a method of using a solid solution in which molecules are aligned without involving inversion symmetry, for example, by applying an electric field. As used herein, the terminology "solid solution" refers to a composition containing a compound exhibiting a nonlinear optical response and a polymer in a homogeneous state. In these methods, organic compounds exhibiting a nonlinear response would be better suited as the nonlinear susceptibility in the molecular state becomes higher. It is known that compounds having a long .pi.-electron conjugated chain are helpful for manifestation of such properties. As referred to in the above-cited references, these compounds naturally show absorption maxima shifted to the longer wavelength side to cause, for example, reduction of transmittance of blue light, which leads to a hindrance to generation of blue light as second harmonic wave. The same disadvantage also occurs in p-nitroaniline derivatives. It is obvious that the efficiency of SHG is greatly influenced by the transmittance at that wavelength, as illustrated in Alain Azema, et al., Proceedings of SPIE, Vol. 400, P186, FIG. 4, New Optical Materials (1983).
Therefore, it has been desired to develop a nonlinear optical material having a high transmittance to blue light. To this effect, substitution of a carbon atom of the benzene nucleus of nitroanilines by a nitrogen atom, etc. has been studied, but fully satisfactory results have not been necessarily obtained.
Under the above-mentioned circumstances, it has been desired to develop a compound which shows high nonlinear optical responses, excellent blue light transmittance, and suitability for forming a solid solution with a polymer.